There is known a gas sensor for determining the concentration of NOx or the like included in exhaust gas or the like of automobiles (refer to patent document 1). The gas sensor is provided with a measurement target gas chamber, a reference gas chamber and a solid electrolyte. The measurement target gas chamber introduces measurement target gases such as the exhaust gases. The reference gas chamber introduces reference gases such as atmospheric air. The solid electrolyte is disposed between the measurement target gas chamber and the reference gas chamber.
The solid electrolyte is composed of a material such as zirconia showing oxygen ion conductivity. A pump electrode and a sensor electrode are formed on the surface of the solid electrolyte at a side of the measurement target gas chamber. In addition, a reference electrode is formed on the surface of the solid electrolyte at a side of the reference gas chamber.
A pump cell for reducing oxygen included in measurement target gases is formed with the pump electrode, the solid electrolyte and the reference electrode. In addition, a sensor cell for determining the concentration of specific gases such as NOx included in the measurement target gases is formed with the sensor electrode, the solid electrolyte and the reference electrode. The sensor cell has sensitivity to oxygen and the specific gases. Therefore, the gas sensor is constructed so that the concentration of the specific gases is determined using the sensor cell after reducing an oxygen concentration included in the measurement target gases using the pump cell.
The pump electrode is composed of Pt—Au alloy or the like having a property of reducing oxygen molecules to oxygen ions. Oxygen molecules included in the measurement target gases are reduced to oxygen ions on the surface of the pump electrode. After that, oxygen ions are discharged from the reference chamber via the inside of the solid electrolyte.
Pores, through which the measurement target gas may pass, are formed inside the pump electrode. Thereby, a contact area between the pump electrode and the measurement target gases is increased, and an ability of discharging oxygen included in the measurement target gases is raised.